A wearable device is light and clings to the body of a wearer, and is therefore a suitable medium for communication between the body of the wearer and the world. According to different product forms, wearable devices may be classified into different types such as watches, wristbands, glasses, armbands, and pendants. Nowadays, wearable devices become increasingly popular in people's lives. For example, a child wears a wearable device and a parent can know the whereabouts of the child in time. In some cases, when a wearable device falls off from a wearer, it indicates that the wearer may be in danger. For example, a criminal forcibly removes a wearable device from a child. In such a case, the wearable device cannot recognize whether the wearer is in danger.